The present invention concerns a chronograph including a device for driving a second counter by a first counter, the second counter being arranged for counting the revolutions made by the first counter from a zero position of the first counter, the drive device including a control finger mounted on the first counter and capable of meshing with a transmission wheel associated with the second counter for driving the latter.
In most cases, the counter referred to above as the first counter is a seconds counter and the second counter is a minutes counter, for example as provided by Swiss Patent No. 571 741 (corresponding to U.S. Pat. No. 3,901,020). However, a similar drive device can be used with different counters, for example for driving an hours counter from a minutes counter.
When the counters of a mechanical chronograph are reset to zero, the rotating wheel sets of the counters have to be able to rotate easily and quickly. The aforementioned control finger is liable to interfere with the toothing of the transmission wheel. A known solution for avoiding this problem is moving the transmission wheel so that it is out of reach of the finger while the first counter returns to its zero position. For example in Swiss Patent No. 678 911 (corresponding to U.S. Pat. No. 5,113,382), the control finger meshes with an intermediate wheel, which is permanently meshed with a wheel of the minutes counter. This intermediate wheel is raised axially by the zero reset command, to move it out of the trajectory of the control finger.
The present invention improves the device for driving the second counter by the first so as to enable it to operate in two opposite directions, namely a direct direction in which the chronograph adds time in a conventional manner, and a reverse direction in which the chronograph counts down, in particular a time which the user has pre-selected by putting at least one of the counters in a start position which defines said countdown time interval.
This double rotational direction creates a new problem linked to the meshing of the control finger with the transmission wheel. Let us assume for the sake of simplification that the seconds counter is initially at its zero position both for a countdown and for conventional counting by addition. If the control finger is fixed to the seconds counter as in the aforementioned Patents, its initial position has to be such that it will move the transmission wheel forward and thus increment the minutes counter at the moment when the seconds counter and the finger finish one complete revolution forwards, which corresponds to 60 seconds. However, this position would not be suitable in the case of a countdown, since the finger would then act on the transmission wheel shortly after the start of its rotation backwards, i.e. already after several seconds, which would produce an erroneous reading for the minutes counter.
The present invention concerns a chronograph with two rotational directions, wherein the aforementioned drive device is made so as to activate the second counter at a precise moment at the end of a revolution of the first counter, in both directions. An additional object is to enable precise angular indexing of the control finger with respect to the other components of the device, in order to adjust precisely the moment at which it will act on the toothing of the transmission wheel. Another additional object consists in arranging the transmission device so as to avoid interference between the control finger and the transmission wheel during pre-selection of the duration of the countdown interval. These objects are intended to be achieved via a simple and compact construction.
According to the invention, there is provided a chronograph of the type indicated in the preamble hereinbefore, characterised in that the first and second counters are capable of rotating in both directions, respectively in an adding mode and in a countdown mode, the second counter being driven forwards or backwards depending on the rotational direction of the first counter, in that the control finger is mounted so as to pivot on a disc linked to a rotating shaft of the first counter and in that the disc and the control finger are coupled to each other by drive means enabling the control finger to pivot on the disc between two stop positions set apart by an angle close to 360xc2x0.
This enables the control finger to occupy one or other of the two stop positions, depending on the direction of rotation, at the moment when it has to start to drive the transmission wheel at the end of a complete revolution of the first counter. The drive means, which define these two stop positions, may advantageously be arranged to allow precise indexing of these positions.
Preferably, the disc is secured to the shaft of the first counter in an indexed angular position with respect to a zero reset heart-piece, by means of which said zero position is defined, and the disc is provided with a spring arranged to keep the control finger free of the transmission wheel when the first counter is at the zero position. With this construction, the aforementioned drive means are preferably adjustable for indexing said stop positions with respect to the zero reset heart-piece.